The invention relates to an oil seal assembly.
In particular, the invention concerns an oil seal assembly for a gas turbine engine, especially but not exclusively an aero gas turbine engine. Except where a lubrication system is designed for total loss, it is usual to take steps to minimise or prevent even small oil leaks especially in locations where a leak may leave a visible, tell-tale mark. Therefore oil seals are normally located where a rotating shaft emerges from a zone containing lubricant. One example of such arrangement occurs at the front of the engine where the main engine shaft extends forwards to drive a low pressure fan or external equipment such as a lift fan.
The three most common types of oil seal used in gas turbine engines are labyrinth seals, brush seals and carbon seals, although in some instances a close tolerance may be considered a seal. Labyrinth and brush seals are not inherently oil tight so that where zero leakage is an objective carbon seals are employed. The performance of all types of seal is improved where a pressure differential is maintained across the seal during operation with the higher pressure on the outside of a bearing housing to ensure that air leaks inwards adding to the effectiveness of the seal. Oil leaking past a seal may cause a safety or fire hazard and anyway causes unsightly oil stains, therefore, it is desirable for several reasons to take all possible measures to eliminate oil leakage.
For ease of reference hereinafter regions or volumes containing lubricant will be referred to as a “wet” zone and a lubricant free region will be referred to as a “dry” zone. So, for example, the interior of a bearing chamber or gearbox may be referred to as a wet zone while the region external thereto will be referred to as a dry zone. Generally, carbon seals are used for sealing oil within bearing chambers and gearboxes. The carbon sealing element is carried in a seal housing located in the bearing housing and bears against a seal runner carried on the rotating element, for example, the shaft. The carbon sealing element rubs against the relatively rotating seal runner theoretically, at least, eliminating the leakage gap and thereby confining lubricant to the wet zone, effectively preventing loss of lubricant into a dry zone. The seal element and the seal runner are mounted concentrically with the carbon seal element located in the wall of the bearing chamber, usually comprising several interlocking segments carried in a mounting capsule to permit limited radial movement. The carbon seal element bear against the surface of the seal runner which, in turn, is supported so that the heat generated by the rubbing action of the seal element on the surface of the runner may be dissipated by directing oil onto an under surface of the runner. Seals and seal runners of this kind are intended to be replaceable at service intervals and so are designed to be demountable.